Why and how I had to make my own definitions

The Body Mass Index calculator has two choices available for deciding "Body Description", either "WHO – CDC" or "halls.md v2".

These choices set the Body Mass Index (BMI) thresholds that determine if BMI values are "overweight", "obese", "normal", "underweight", etc. The preceding page described the WHO – CDC criteria. This page describes the "halls.md v2" criteria.

The halls.md v2 BMI criteria

by Steven B. Halls, MD FRCPC

For Children, the halls.md v2 method uses the same Body Mass Index for Age chart criteria as the WHO & CDC. In other words, for children there is no difference between the two methods. The 85th percentile of BMI for Age is the Overweight threshold, and the 95th percentile is the Obesity threshold. For further information, see the boys BMI chart or the girls BMI charts, which illustrate these thresholds.

For Adults, the halls.md v2 method uses the following thresholds:

"Marginally Overweight" is Body Mass Index values that are higher than a threshold of approximately 24 to 25 kg/m2,

"Overweight" is Body-mass-index values higher than approximately 25 to 27,

"Obesity" threshold is approximately 29 to 32,

"Underweight" threshold is BMI values below approximately 18 to 20.7,

Each threshold varies by Age and Gender.

The charts below show how these threshold values vary with Age and Gender.

The lines are curved, not flat. They allow older people to be heavier.

Is this Science?

It’s educated guessing, based on an mix of scientific data and some assumptions. The "halls.md v2" method cannot possibly be perfectly correct. To some people, when things are not perfectly correct, they are wrong. However, the WHO & CDC criteria are not perfectly correct either.

The name "halls.md v2" indicates "version 2", which replaced version 1 in November 2002. The WHO – CDC method was the default for the Body Mass Index calculator for many years, but now the halls.md v2 is default.

Principles of halls.md BMI criteria

Principle #1. BMI criteria should change according to age in adults.

Do 60-year old adults resemble 20-year olds? Of course not. It is natural for body mass index to change with age, and it’s healthy.

If you are over 50, must see this link.

Next, consider skinny men and women at the 10th percentile of BMI. These are people who tend to remain slim throughout their lives, but their BMI changes with age. In fact all other percentile lines change with age too.

The data points shown in the graph (above) are from various big BMI surveys, showing just the data from the skinny people. The solid purple trendline curve shape is used in the halls.md v2 model, as the shape of the Overweight and Obesity threshold lines. This curve shape is an assumption. I call this the "slim changes proposal". Stated in words, the halls.md model assumes that people naturally gain about 1.1 kg/m2 between age 20 to 30, gain 0.6 kg/m2 between age 30 to 40, gain 0.4 kg/m2 between age 40 to 50, etc.

This seems slightly more scientific than the old "rule of thumb" to allow BMI to increase by 1 extra kg/m2 per decade for Ages 40 and over.

Principle #2. The WHO & CDC criteria are best suited to young adults.

Knowing that BMI changes with Age, perhaps you are wondering if the WHO & CDC intended their "Overweight" criterion of BMI = 25, to apply to Young adults or Average adults or Middle-Aged adults? The answer is… (drumroll)… for 18 year olds.

Therefore, in order maintain some consistency with the CDC, the halls.md model also assumes that a BMI of 25 is overweight for 18 year olds, and the slim changes proposal curve-shape, is shifted upward to cross a BMI of 25 at age 18.5, (allowing for slight differences between men and women).

The halls.md model is a reasonably close fit to the WHO & CDC model for young adults, who will get very similar results with either model. But for older adults, the halls.md model differs from the WHO – CDC model.

I used BMI of 25 for 18 year olds as a “given”, just like the CDC.

You’re not an anarchist.

Principle #3. Men and women’s Overweight BMI differs by 2.0 kg/m2.

The "overweight" threshold for 18-year olds is the same number (25 kg/m2) in men and women, to preserve some compatibility with the CDC. But the number is not the same in younger children and should not be the same in older adults. The halls.md v2 model assumes that adult men are allowed to be heavier than women by 2.0 kg/m2). This arbitrary number came from averaged results from many sources of published evidence. (click this link to read more).

Principle #4. Nobody likes being called overweight.

We avoid calling kids overweight, unless it is really obvious. Preserving self-esteem is also important, even for adults. If the overweight threshold is too low, it will label nearly 100% of the people who are truely overweight, but also falsely label MANY normal people too. (That’s what the CDC criteria does to adults). These "false positives" should be avoided, even if it reduces the number of "true positive" people labelled as overweight. Thus, the halls.md v2 criteria are designed to have higher specificity than sensitivity.

Various target points for BMI in middle-aged and elderly adults were gathered from a review of published scientific articles. These target points are coincidentally very close to the halls.md v2 definitions, and provide additional validation of the appropriateness of the halls.m v2 criteria. These target points are discussed on this page.

Soon I think, I’ll make Version 3, and allow BMIs to be even higher in older people.

Principle #6. “Marginally Overweight” is a gentler phrase.

Before the WHO international definition of Overweight at BMI of 25, an earlier standard (based on NHANES II2) used the phrase "Marginally overweight" for adult BMI values at or above 25.8 in women and 26.4 in men.

The recent WHO international definition has lowered those BMI values to 25 and dropped the word "Marginally".

The halls.md model also adjusts for women’s height

Another feature of the halls.md model, is an extra adjustment of the Overweight criteria based on a womansheight. In Adult men, median Body Mass Index is stable ( horizontal lines ) at all heights from short to tall. But for Adult women, short women naturally have a higher BMI than tall women4. This is true at ages 30 or higher, even for slim (10th percentile) women. It is probably related to varying leg length. Therefore, the Body Mass Index calculator makes an extra adjustment for this factor.

( These charts may seem confusing, but they contain data not available anywhere else, so I felt it was important to show them. The charts were derived from the NHANES III dataset1. My thanks to John Hanson, MSc for his assistance with data manipulation.)

What’s missing?

The halls.md v2 criteria don’t adjust for race or nationality differences. There is some published medical literature available on this topic and I may try to implement it in the future, if demand exists. Also missing is an adjustment for muscularity, so that body builders don’t get insulted when their BMI suggests they are overweight or obese.

After I decided to try to create my own BMI criteria, I realized that there are a lot of potential areas for improvement. Since I am single person, not a committee or a famous national organization, I have a certain degree of freedom to try new ideas.

Which reminds me, to remind you, that the halls.md v2 criteria are the opinion of one person.

You can help.

Are the halls.md v2 criteria too lax regarding Overweight? I’d like to find out from you. Email me.

If you are overweight or obese, (and if most people think so too), but if halls.md v2 indicates you are in the Normal Range, then please email me, to tell me about it. Send email to [email protected] I’m looking forward to making Version 3.